In the cathodic electrocoating art, dispersions (usually aqueous) containing cationic resins are formed from resins having either salt groups with separable anions or groups ionizable to form cations by reaction with a suitable ionizing species (usually an acid) present in the dispersion medium. The cationic resins for cathodic electrocoating usually have either protonated amine groups or quaternary ammonium ions as their cations. The ionization of these cationic resins, together with the presence of water-soluble counter-anions, stabilizes the dispersion of these resins and they in turn can act as dispersing agents for other non-ionized coating components such as other resins, cross-linking agents, pigments, etc. The cationically dispersed components are attracted to a conductive substrate in contact with the dispersion when that substrate is charged as a cathode and are deposited thereon by at least partial electrical discharge of the cationic resin at the surface of the substrate-cathode.
Cationic resin dispersions suitable for cathodic electrocoating have been based on a variety of resin types, including aminated epoxy resins formed by reaction of polyamines with epoxy resins and dispersed in aqueous medium by ionization with a variety of water-soluble acids. Thus U.S. Pat. No. 3,963,663 (Sekmakas) teaches adding cationic groups to urethane-epoxy resins by reaction of the epoxy functionality with diprimary amines to give a mixture of primary and secondary amine groups for ionization with water-soluble acid; phenoplast resins are shown as cross-linkers and aminoplast resins are specifically contra-indicated for cathodic electrocoating use (cf. column 3, lines 57 to 68). (In an earlier patent, U.S. Pat. No. 3,891,527, Sekmakas disclosed use in cathodic electrocoating of aqueous cationic dispersions of aminated epoxy-urethane-ether resins formed by exhaustive amination of the epoxy functionality of diepoxide-urethane-ether resins with mono-secondary amines, optionally codispersed with aminoplast resin curing agent and also optionally having carboxylic acid groups on the aminated epoxy-urethane-ether resin.) U.S. Pat. No. 3,947,339 (Jerabek et al) discloses cathodic electrocoating compositions comprising self-curing resins which have primary amine groups attached by reaction of a polyamine having both a secondary amine group and also ketimine-blocked latent primary amine groups (liberated upon dispersion in aqueous medium) with epoxy resins having both blocked isocyanate groups and hydroxyl groups, the resulting resin being dispersed in water by at least partial ionization of the amine groups with a water-soluble acid. U.S. Pat. No. 2,772,248 (Lieberman et al) discloses aminated epoxy resins which are reaction products of epoxy resin with a polyamine containing two or more primary or secondary amine groups and are soluble in aqueous acids, but neither electrocoating use nor inclusion of a cross-linker is suggested.
Aminoplast resins are well known as curing agents for resins having active hydrogens. Both U.S. Pat. No. 3,471,388 (Koral) and U.S. Pat. No. 3,894,993 (Blanks) disclose cathodic electrocoating compositions which contain polyamine-aminated epoxy resins having both primary and secondary amine groups and also comprising etherified formaldehyde-aminotriazine adducts as curing agents, but neither suggests having carboxylic acid groups associated with the aminoplast resin for catalyzing cure. U.S. Pat. No. 3,519,627 (Coats et al) discloses etherified benzoguanamine- or melamine-formaldehyde adducts having aliphatic carboxylic acid groups for use as curing agents with anionic resins in anodic electrocoating compositions, the carboxylic acid functionality of the aminoplast being described only as aiding in the codeposition of the aminoplast on the anode-substrate; there is no suggestion of any use with cationic resins or in particular in cathodic electrocoating.